Hydraulic injection molding machines generally have a hydraulic drive device. This hydraulic drive device is provided with: a screw drive portion having an injection cylinder for moving a screw in an injection device forward or backward and an oil motor for rotating the screw; and a hydraulic drive portion for driving this screw drive portion. The hydraulic drive device is also provided with back pressure control means for applying a predetermined amount of a back pressure to the screw at the time of measurement when the screw is moved back or at the time of a sack back process. As this back pressure control means, a pressure control valve is used that is connected to the rear oil compartment of the injection cylinder. In this case, when the back pressure is applied, the back pressure (hydraulic pressure) is detected by a pressure sensor, and the pressure control valve connected to the rear oil compartment of the injection cylinder is controlled by feedback such that the back pressure detected reaches a predetermined target value. Thus, a molding machine controller is provided with a feedback control circuit for controlling such a back pressure by feedback (see patent document 1 for example).
A control valve is also known in which such a feedback control circuit is integrally fitted to a pressure control valve and this facilitates adjustment of an electrical system when the pressure control valve is used and achieves high-speed response. In patent document 2, there is disclosed a proportional solenoid control valve device including an oil immersion electromagnetic solenoid plunger device that produces a mechanical output proportional to an excitation current at a movable portion, a fluid control valve device that controls the pressure or the amount of fluid by being displaced by the mechanical output through a spring force and a current driver device that feeds, as the excitation current, an output current corresponding to a previously applied input signal to the electromagnetic solenoid plunger device, in which the movable portion of the solenoid plunger device is formed with a movable element showing substantially no damping characteristic when it moves within oil and in which the current driver device is provided with a current control circuit, in a feedback system, that limits the increase of the excitation current so as to prevent vibrations caused when the movement of the movable element is stopped as the excitation current rises.